This effort is concerned with the synthesis and characterization of a new type of polymer architecture, polyrotaxanes. These structures consist of linear polymers which have been threaded through the cavities of a large number of cyclic molecules. The polyrotaxanes consist then of a physically bonded set of molecules; there is no covalent bond between the linear and cyclic species. We have utilized aliphatic crown ethers nearly exclusively as the cyclic molecules. The linear polymers have included polyesters, polyurethanes, polyamides, polystyrene, poly(phenylene vinylene), poly(ether ketone)s and poly(ether sulfone)s. Significant changes in properties result from the physical linkage of the cyclic species into the polymer; these include enhanced solubility, solvent induced changes in hydrodynamic volume, alteration of the glass transition temperature and introduction of crystalline domains of the macro-cyclic component. These polymeric systems, accessible by step growth (condensation), free radical, anionic, or cationic techniques, possess unique physical and chemical properties. Molecular engineering should provide materials useful in a number of application, such as adhesives, composite matrices, polymer blending and toughening, energy and electron transfer, controlled-release membranes, and perhaps further into the future, molecular-level electronic devices.